This invention relates generally to the use of seismic data processing and interpretation and more specifically to the combination of multiple seismic signals, the improvement of signal to noise ratios, and the modulation and demodulation of seismic information.
Originally, acoustic and seismic information was gathered in a seismic survey for the primary purpose of determining arrival times of seismic events, thus gaining knowledge of the subsurface. These seismic surveys utilized either seismic motion or pressure sensitive sensing transducers within an array that was deployed in or on the earth or in a body of water, respectively, for the purpose of recording seismic acoustic information. The seismic information that is output and recorded from one of these arrays consists typically of one signal. In these processes, the output is of a type of amplitude modulated signal that is recorded for processing and subsequently used to interpret subsurface structure and composition.
Such a use of an amplitude modulated signal imposes significant constraints on the improvement of signal to noise characteristics of the returning earth signal and further constrains the ability to determine subsurface structure and composition. It has long been recognized that there are many forms and types of noise, both coherent and random, which are sensed and recorded in a seismic system. Many types of noise are significant in certain frequency bands commonly narrower than the total bandwidth of sensed and recorded seismic data. For example, ground roll is commonly a low frequency seismic energy that travels directly from source to receiver along the ground. Ground roll commonly lies in the frequency range of 10-30 Hz. Likewise, air blast is the energy that travels through the air from source to receiver. This type of noise commonly has frequency characteristics in the 40-80 Hz range. In addition, there are many other types of noise which have characteristic signatures.
Amplitude modulated signals restrict the use of more detailed information such as amplitude, phase, frequency, and interval velocity because of superposition of signal and, especially, superposition of noise. As noise, an undesirable signal, and the desired seismic signal are recorded by one type of receiver, they are superimposed. The ability to separate these individual signals is limited to processing the amplitude modulated output signal received by this type of receiver. In addition, phase characteristics of the desired signal and the noise signal are further constrained because of superposition. In fact, information is commonly recorded with one type of receiver whose phase response is nonlinear.
Thus, the use of an output signal characterized by an amplitude modulated signal imposes significant constraints on the improvement of signal to noise characteristics of the returning earth signal and further constrains the ability to determine subsurface structure and composition. The ability to further improve signal to noise is further contrained by the inability to solve harmonic distortion or the representation of two or more superimposed harmonics. This is also true for the processing and interpretation of three-component seismic information.
Originally, seismic acquisition constractors used geophone and hydrophone arrays with identical individual geophones and hydrophones consisting of transducers of one natural or resonant frequency all being connected by a flexible wire for the purposes of electrical communication and, in the case of hydrophones, position maintenance through drag. A serious impediment exists in fast moving bodies of water such as rivers and tidal areas.
Seismic acquistion in the marine environment has typically utilized a number of hydrophone arrays strung together by wire and towed behind a boat. Movement of the boat deploys a drag type seismic cable containing the hydrophones and hydrophone arrays in a straight line. Tidal marine conditions exist in coastal areas where strong horizontal ocean currents can easily destroy seismic equipment. Fast moving bodies of water like rivers also have very strong currents that have made previous attempts to acquire seismic reflection data in these water environments generally unsuccessful.
Navigation and position maintenance of a drag type seismic cable containing hydrophone arrays in tidal marine and river environments is very treacherous. Strong currents easily disrupt loosely connected geophone arrays strung together by wire. Sheltering of marine and land geophones from noise producing water and wind turbulence is limited.
Exemplary of the prior art is U.S. Pat. No. 2,688,124 to Doty et al., which is directed to a process of emitting signals having different frequency components. The process of this reference is subject to the undesirable collection of the controlled input of multiple frequencies by a single type of detector. Additionally, benefits are limited to correlation and determination of the time-phase relationship of a travel time path through the process of time multiplexing.
U.S. Pat. No. 4,875,166 to Carroll et al. is directed to increasing the bandwidth of recordable seismic information. The system taught by this reference is subject to sensing all signals within a single type of sensor early on thus limiting signal to noise improvement.
U.S. Pat. No. 5,289,433 to Cowles et al. teaches a method and apparatus for faithfully recording borehole acoustical signals. An attempt is made to reduce noise by providing a semi-rigid reciever array for the purpose of decoupling certain types of noise. A plurality of different natural or resonant frequency sensors are deployed, the output of which is subsequently combined for the purpose of optimizing multifrequency output and bandwidth. This reference teaches that noise is eliminated mechanically by decoupling, much like the well known surface technique of array distribution.
U.S. Pat. No. 5,231,611 to Laznicka is directed to a remotely deployable, unpowered sonar sensor array. This reference discloses the use of a hydrophone array serving as an acoustic phase array based on the known phase relationship between different natural or resonant frequency transducers.
U.S. Pat. No. 4,405,036 to Brede is directed to an apparatus for stabilizing a drag type seismic cable containing hydrophone arrays through the use of a boat at one end of the tow line and a data collection truck based on the shore. The apparatus of Brede is subject to undesirable attenuation of the hydrophone signal due to the reflection coefficient of the seabed.
U.S. Pat. No. 4,463,451 to Warmack teaches a way to stabilize and maintain the relative position of a single geophone in a water covered area using a recording float and an elaborate tension filter, which is expensive and difficult to deploy.
U.S. Pat. No. 2,738,488 to MacKnight is directed to a single drag type cable employing single geophone attachments, which have questionable coupling and position control in tidal marine areas and fast moving rivers.
U.S. Pat. No. 5,014,813 to Fussell is directed to a waterproof housing for single seismic sensor typically used in marshes and other areas of quiet water. The typical marsh geophone is elongate and is made to be coupled or positioned in the mud. Groups of geophones are connected by wire for the purpose of electrical communication. Planted singly, these geophones arrays can easily become decoupled and are ineffictive in tidal marine areas and rivers.
U.S. Pat. No. 4,138,658 to Avedik is directed to a complex pickup, comprising a detachable frame, a hydrophone and two geophones that are used in water depts of 100-200 meters in connection with refraction surveys. This arrangement suffers from poor earth coupling, since the geophones are not individually planted in the earth. Only the frame is directly coupled to the earth through the three feet that are provided on the underside thereof. Therefore, the geophones sense refractions through the frame, rather than directly from the earth.
Seismic acquisition on land has in the past utilized a plurality of geophone arrays strung together by wire for the purpose of better sampling, notching the resulting frequency spectrum, and electrical communication to a multichannel recording unit. Individual geophone arrays normally consist of a plurality of geophones that are planted in spaced groupings of 12-24 geophones over distances of 55-440 feet by unskilled operators who have little regard for accurate positioning and proper orientation of the geophones. The distribution of the geophone group is a further attempt to eliminate ground roll by notching the data set. Terrain changes are not usually taken into account. When these arrays of geophones are planted on hillsides, plane reflection waves coming from different directions impinge on the group array at different angles, thereby causing misalignment of the response signals.
In an attempt to improve data quality, three-dimensional geophones have been used to measure motion in three orthogonal directions. Each three-dimensional geophone typically comprises three separate unidirectional geophones that are oriented for three-dimensional pickup and housed in a single enclosure, as typified by the geophone described in U.S. Pat. No. 5,010,531 to McNeel. These devices are disadvantageous in that they are difficult to repair, require strict horizontal placement, and contain identical transducers.
It is therefore an object of the present invention to provide a seismic sensor array containing a plurality of seismic sensors in a configuration that results in a more accurate measurement of seismic reflection energy.
It is another object of the present invention to provide a seismic exploration apparatus that employs an improved communication system from source to receiver.
It is another object of the present invention to provide a seismic exploration method in which modulated seismic signals are used to obtain a more accurate representation of the subsurface.
It is another object of the present invention to provide a seismic exploration method in which improved noise rejection and isolation results in improved imaging, resolution, and interpretation of subsurface structure and composition.
It is another object of the present invention to provide a seismic exploration method resulting in a more accurate representation of seismic frequency of the recorded seismic information for the purpose of improving imaging, resolution, and interpretation of subsurface structure and composition.
It is another object of the present invention to provide a seismic exploration method resulting in a more accurate representation of seismic phase of the recorded seismic information for the purpose of improving imaging, resolution, and interpretation of subsurface structure and composition.
It is another object of the present invention to provide a seismic exploration source system for use in an angle modulation seismic survey.
It is another object of the present invention to provide a seismic exploration apparatus employing a receiver system for use in an angle modulation and demodulation seismic survey.
It is another object of the present invention to provide a seismic exploration method apparatus employing an improved seismic communication system for use in isolating and defining both a desired seismic signal and an undesired noise signal.
It is another object of the present invention to provide a seismic exploration method in which an angle modulated seismic signal is processed at the array site.
It is another object of the present invention to provide a seismic exploration method in which an angle modulated seismic signal is processed at the recording site.
It is another object of the present invention to provide a seismic exploration method in which an angle modulated seismic signal is processed interactively during an interpretation stage.
It is another object of the present invention to provide a seismic exploration apparatus in which deployment and location of seismic sensors is robotized.